56% of Delhi population has antibodies, fueling India's COVID cases mystery

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More than half the population of India’s capital city has contracted and recovered from COVID-19, according to the results of a government serological survey released this week.

Health authorities took blood samples from 28,000 people across 11 districts in Delhi and found that 56% tested positive for coronavirus antibodies, the proteins that the immune system produces in response to an infection.

The results indicate that far more people in Delhi have contracted COVID-19 than the official case count of around 635,000. Delhi’s population was 16.8 million in the last official census in 2011, with a current estimated population of 23 million, meaning that around 10 million city residents may have coronavirus antibodies, according to the survey.

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The study also suggests that Delhi could achieve herd immunity, where enough people are immune to the virus that it stops spreading among the population, which is what mass COVID-19 vaccination campaigns aim to achieve.

Delhi’s health minister Satyendar Jain appeared to endorse that idea on Twitter on Tuesday evening when he shared the results of the survey and said, “Delhi has largely won over COVID, though we should continue to practice [COVID-19] appropriate behavior.”

The idea of deliberately advancing herd immunity through natural infection is controversial because it places vulnerable segments of the population at risk, and because herd immunity has largely been studied in the context of immunity through vaccinations, not natural immunity. If Delhi has achieved herd immunity, it did so inadvertently, driven by waves of infection that occurred throughout 2020, including large outbreaks when the government loosened lockdown restrictions last summer.

An August survey of Delhi’s population, conducted just before a massive nationwide spike in infections, estimated that around one quarter of Delhi’s population had coronavirus antibodies.

The most recent survey took place between Jan. 15 and Jan. 23. It was the fifth seroprevalence survey carried out in Delhi since the start of the coronavirus outbreak, and the largest government survey so far, according to Jain.

On Monday, Delhi reported three deaths and 32 hospitalizations from COVID-19, both the lowest daily figures for those metrics in 10 months. On Tuesday, India recorded the lowest number of nationwide daily new cases in eight months.

Health experts around the world are still puzzling over India’s slowing rate of infections. Widespread natural immunity is one potential cause, but scientists caution that more surveys are needed. Plus, scientists say, it’s still not clear how long COVID-19 antibody resistance lasts, and achieving herd immunity would require that a large share of the population—at least 60% to 80%—test positive for antibodies.

Public health experts in India told local media on Wednesday that while the survey results mean Delhi is “unlikely” to experience another large spike in COVID-19 infections, widespread natural immunity doesn’t rule out the need for mass vaccinations.

India’s government launched a nationwide vaccination program in January with the aim of inoculating 300 million people in the next eight months.